Timing of deformation phases within the South Finland shear zone, SW Finland

• Published in: Precambrian Research
• Main Authors: Torvela, Taija, Mänttäri, Irmeli, Hermansson, Tobias
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier 2008
• Subjects: Geology; shear zone; reactivation; deformation; SIMS; ID-TIMS; Ar-40/Ar-39; palaeoproterozoic; Fennoscandia
• Online Access: https://lup.lub.lu.se/record/621748; https://doi.org/10.1016/j.precamres.2007.08.002

Abstract in UndeterminedThe Palaeoproterozoic Svecofennian crust in southern and central Fennoscandia was established about 1.8 Ga ago after a prolonged history of accretion and intrusion. During late stages of the Svecofennian orogeny, deformation was partitioned into several crustal-scale shear zones in present-day Finland, Sweden and Estonia. One such major ductile deformation zone, 'the South Finland shear zone' (SFSZ) extends for almost 200 km through the (A) over circle land archipelago in southwestern Finland, and further along the southern and southwestern coast of Finland. This more than a kilometer wide transpressional zone appears to have been repeatedly reactivated. The deformation started with a period of regional, ductile dextral shearing of igneous rocks, producing striped granodioritic and tonalitic gneisses. The ductile phases are locally overprinted and followed by ductile to semi-ductile deformation evidenced by mylonite zones of variable width. The last stage of tectonic activity along the shear zone is recorded by pseudotachylites. Within this study, we dated zircons (SIMS U-Pb) and titanites (ID-TIMS U-Pb) from eight rock samples, and two pseudotachylite whole-rock samples (Ar-40/Ar-39) in order to reconstruct the deformation and (re)activation history of the shear zone.The results suggest that the medium-grained gneisses underwent three distinct deformation phases separated by time intervals without regional deformation. The ductile deformation within the study area initiated at similar to 1.85 Ga. A second, more intensive deformation phase existed around 1.83 Ga, by which the shear zone was already well developed. Finally, the last ductile event is recorded by similar to 1.79 Ga metamorphic titanites in relatively granoblastic granitoid gneisses that nevertheless already display protomylonitic textures, suggesting the initiation of large-scale mylonitisation around or soon after this time. The age of a pseudotachylite sample and, hence, the brittle deformation is bracketed between 1.78 ...